I just went to Google Play to look into it. They don't list costs. They do mention Free, Member, and Premium subscription levels, but nothing about costs.
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This kind of behavior just leaves an initial bad taste in my mouth. They may indeed have a good product, but their corporate practices leave something to be desired. If they go that far out of their way to avoid the question about what it costs, the answer must be unfavorable for them. The company must see some advantage in initially not telling you what it costs, and that makes me very wary of their motives. It's like websites that hide shipping charges until after you've already gone through the shopping cart process, entering your address, entering your CC number, etc. I don't order from places like that either.